The present invention relates to photographic silver halide emulsions exhibiting enhanced sensitivity and improved sensitivity at low intensity exposure.
Silver halide photographic light sensitive materials (hereinafter, also denoted simply as photographic materials) are said to be mature products having a high level of completeness, while various performance factors such as high sensitivity, enhanced image quality and improved storage stability are required and recently those requirements have been raised to higher levels. Specifically, with regard to high sensitivity and enhanced image quality, further enhanced performance is required to maintain superiority of silver halide photographic materials in view of recent technical progress in digital cameras.
To achieve higher sensitivity and enhanced image quality, there has been studied a technique for enhancing the ratio of sensitivity to grain size for respective grains in a silver halide emulsion (hereinafter, also denoted simply as an emulsion).
It is commonly known that silver halide grains contained in a silver halide emulsion have, in general, various shapes. Examples thereof include regular crystal silver halide grains such as cubic, octahedral or tetradecahedral grains, tabular silver halide grains having a single twin plane or plural parallel twin planes, and tetrapod-like or needle-like silver halide grains having non-parallel twin planes. Specifically, tabular silver halide grains (hereinafter, also denoted simply as tabular grains) are supposed to have the following advantages as photographic performance:
1. The ratio of grain volume to grain surface area (hereinafter, also denoted as its specific surface area) is relatively high, allowing a large amount of a sensitizing dye to be adsorbed onto the surface so that spectral sensitivity is high relative to intrinsic sensitivity;
2. When tabular grains containing emulsion are coated and dried, the tabular grains are arranged parallel to the support surface and thereby, the coating layer thickness can be reduced, leading to enhancement of sharpness of the photographic material;
3. Light scattering due to silver halide grains is relatively low, resulting in images with high resolution;
4. Sensitivity to blue light (intrinsic sensitivity) is relatively low so that when used in a green-sensitive or red-sensitive layer, the yellow filter density can be reduced or the yellow filter can be entirely removed from the constitution of a photographic material; and
5. In cases when having achieved the same sensitivity as commonly known other type grains, the characteristic grain shape results in a reduced silver coating amount, leading to enhancement of sensitivity/graininess ratio and superior resistance to natural radiation.
As prior art relating to tabular grains, preparation methods and utilizing techniques thereof are described in U.S. Pat. Nos. 4,434,226, 4,439,520, 4,414,310, 4,433,048, 4,414,306 and 4,459,353; JP-B Nos. 6-43605, 6-43606, 6-214331 and 6-222488 (hereinafter, the term, JP-B refers to Japanese Patent Publication); JP-A Nos. 6-43605, 6-43606, 6-214331, 6-2224888, 6-230493 and 6-258745 (hereinafter, the term, JP-A means Japanese Patent Application Publication).
To effectuate the foregoing advantages of tabular grains, it is effective to employ tabular grains having a higher aspect ratio. A technique used in combination with high aspect ratio tabular grains, as a technique for enhancing the ratio of sensitivity to grain size of silver halide grains is introduction of dislocation lines into those silver halide grains. Techniques for introducing dislocation lines, which are described in JP-A No. 63-220238, 1-102547, 6-27564 and 6-11781 are a sensitivity enhancing technique frequently employed in the photographic art. A tabular grain emulsion having a high aspect ratio and including dislocation lines may be said to be the arrival point of achievement for a high speed silver halide emulsion.
Recently, a compound exhibiting the function of injecting at least two electrons into silver halide through photoexcitation by a single photon is noted as a means for enhancing the sensitivity of a silver halide emulsion. In addition to doubling the number of electrons obtained by one photon, the compound contributes to an enhancement in sensitivity of the photographic emulsion by minimizing the loss process due to recombination of the formed electron with the oxidized dye or a positive hole. The function and reaction mechanism of the compound are detailed in Nature, 402, page 865 (1999); and J. Am. Chem. Soc., vol. 122, page 11934 (2000). There are also disclosed techniques employing this compound in U.S. Pat. Nos. 5,747,236, 6,010,841, 6,054,260, 6,153,371; and JP-A No. 11-237710. It was further found by the inventors of this application that addition of an organic compound capable of forming a cation with a valence of (m+n), i.e., an (m+n)-valent cation, from an n-valent cation radical with an intramolecular cyclization reaction (in which xe2x80x9cnxe2x80x9d and xe2x80x9cmxe2x80x9d are each an integer of 1 or more) resulted in a similar function.
Although techniques for enhancing sensitivity of silver halide grains are known as described above, there has been required a technique for improving sensitivity at low intensity exposure, which is needed at the time of photographing at a slow-shutter speed or in astrophotography, as well as sensitivity at ordinary intensity exposure.
Accordingly, it is an object of the present invention to provide a silver halide photographic emulsions exhibiting enhanced sensitivity and improved sensitivity at low intensity exposure.
The foregoing object can be accomplished by the following constitution:
1. A silver halide emulsion comprising silver halide grains, wherein at least 50% of total grain projected area is accounted for by tabular grains having an aspect ratio of 10 to 100 and at least 50% by number of total grains is accounted for by tabular grains having at least 30 dislocation lines per grain in the fringe portion of the grain, and the emulsion contains a compound having a function of permitting injection of at least two electrons into silver halide via photoexcitation by a single photon;
2. the silver halide emulsion described in 1. above, wherein the compound having a function of permitting injection of at least two electrons into silver halide through photoexcitation by a single photon is an organic compound capable of forming an (m+n)-valent cation, from an n-valent cation radical with an intramolecular cyclization reaction, in which xe2x80x9cnxe2x80x9d and xe2x80x9cmxe2x80x9d represent an integer of 1 or more;
3. the silver halide emulsion described in 1. or 2 above, wherein the silver halide grains have a shallow electron trap within the grain;
4. the silver halide emulsion described in 1. or 2. above, wherein the silver halide grains have a hole trap center within the grain.